JP3262994B2 - High hardness stainless steel with excellent hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high hardness stainless steel which improves hardenability, which is a problem in stainless steel having high hardness, and is particularly suitable for induction hardening.

[0002]

2. Description of the Related Art Generally, SUS4 having high hardness for applications such as corrosion-resistant bearings and shafts that require corrosion resistance and wear resistance.
High C-16Cr martensitic stainless steel such as 40C and medium C-13Cr martensitic stainless steel such as SUS420J2 are used. SU in this
S440C is 58HRC (653HV) by normal quenching and tempering.
(Equivalent) or higher, but there is a problem that in the annealed state, there are many coarse eutectic carbides and workability is poor. In contrast, SU
Since S420J2 is lower in both C and Cr than SUS440C, there is no eutectic carbide and the workability in the annealed state is good, but on the contrary, the hardness in the quenched and tempered state is low, so the wear resistance is inferior. There is a problem that the life is shortened. Compared to the conventional martensitic stainless steel as described above, a 0.7C-13Cr-based alloy that achieves both high hardness in a quenched and tempered state and good workability in an annealed state by reducing coarse eutectic carbides. Martensitic stainless steel has been developed.

However, in recent years, there has been a strong demand for conversion to induction hardening, in which in-line heat treatment can be performed in order to reduce process costs such as heat treatment and processes. However, the above-mentioned steel types are basically made of chromium carbide to secure strength. Therefore, the solid solution of carbides in the matrix is slow and hardenability is poor, and the short-time heat treatment such as induction hardening cannot secure the high hardness normally obtained by quenching and tempering. ing.

[0004]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and enables a reduction in process cost, while improving hardenability while maintaining good workability in an annealed state.
An object of the present invention is to provide a high-hardness stainless steel in which induction hardening is greatly improved so that 58 HRC (653 HV) or more can be obtained even by induction hardening.

[0005]

Means for Solving the Problems The inventors of the present invention replace the carbon, which is the most important component for determining the quench hardness, with nitrogen, which is the same solid solution type element, and the composition of chromium carbide becomes chromium carbonitride. things and becomes very can finely dispersed than chromium carbide alone, in addition more Mn for promoting the solid solution of low temperature and short time its fine carbonitrides, and optimally control the Ni amount a ₁ transformation point In addition to adding tens of ppm of B, which improves the quench hardening ability while reducing the quenching, the hardenability of this component system is greatly improved, and the induction hardening is achieved by controlling the total amount of C + N. 58 hardness in processing
It has been found that HRC (653HV) or more can be secured. Furthermore, since the carbides are finely dispersed, the carbides are less likely to be crack initiation points due to separation at the interface, and workability during annealing can be ensured.

The gist of the invention is as follows: (1) By weight%, C: 0.40 to 0.70%, Si:
0.5% or less, Mn: 0.5 to 2.0%, Ni: 0.2
5 to 1.0%, Cr: 11 to 14%, Mo: 1.5% or less, N: 0.05 to 0.30%, B: 0.001 to 0.
High hardness stainless steel with excellent hardenability, characterized by comprising 015%, the balance being Fe and unavoidable impurities. However, C + N: 0.55 to 0.85% (2) In addition to the above component ranges, V, W, and Ti alone are added in 0.1%.
It is a high hardness stainless steel excellent in hardenability, characterized by containing 50% or less, or a combined total of 0.70% or less.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the amount of each chemical component of the steel of the present invention will be described below. C is an element effective for increasing the hardness after quenching and lowering the A ₁ transformation point. However, when added in a large amount, coarse eutectic carbides in an annealed state are present and workability is significantly impaired. If it is less than 0.40%, quenching and tempering hardness is 58 HRC even if predetermined nitrogen is added.
(653 HV) or more cannot be secured. Also, 0.7
If it is added in excess of 0%, coarse eutectic carbides will precipitate and workability will be significantly impaired. Therefore C is 0.40
-0.70%.

[0008] Si is an element useful as a deoxidizer,
If it is added in excess of 0.5%, annealing will increase the hardness and impair workability. Therefore, the upper limit is 0.5
%. Mn improves hardenability, increases the tendency to form austenite, and lowers the A ₁ transformation point. Thereby, solid solution of carbonitride in the matrix can be promoted and hardenability can be improved, but if it is less than 0.5%, its effect is not sufficient due to the balance with other components. Also,
If added in excess of 2.0%, annealing will increase the hardness and impair workability. Therefore, the Mn content is limited to 0.5 to 2.0%.

Ni enhances austenite formation tendency, and A
₁ Lower the transformation point. Therefore, it has the effect of promoting solid solution of the carbonitride in the matrix during quenching. Ni is an electrochemically noble element, and when added, electrochemically acts on the entire structure in a stable direction to improve corrosion resistance. These effects appear when added at 0.25% or more. However, if added in excess of 1.0%, the martensitic transformation onset temperature (Ms point) is reduced, and residual austenite is formed after quenching, which has the effect of reducing quenching hardness and increasing annealing hardness. Therefore, the workability in the annealed state is significantly impaired. Therefore, Ni is 0.25 to 1.
0%.

[0010] Cr is an indispensable element for ensuring the corrosion resistance of stainless steel, and it is necessary to add 11% or more to ensure the corrosion resistance. But,
If it exceeds 14%, the tendency to form hard carbonitrides is increased, and the workability in the annealed state is impaired, and the carbonitrides are liable to be coarsened, so that the solid solution in the matrix is delayed and the quenching hardness is reduced. In addition, the effective Cr content in the matrix decreases, resulting in impaired corrosion resistance. Therefore, the Cr content is limited to 11 to 14%. Mo has the effect of enhancing the corrosion resistance by strengthening the Cr oxide film (passivation film) that maintains the corrosion resistance of stainless steel. However, when added in a large amount exceeding 1.5%, coarsening of the carbide in the annealed state is promoted and the workability is deteriorated because the hardness of the annealing is improved. Therefore, the upper limit of Mo is set to 1.5%.

N is an element essential for securing quenching hardness by forming a solid solution in the matrix together with C. When N is added, compared to a material having the same amount of C as C + N of the material, the refinement of carbides is promoted and the peak temperature of quenching hardness is lowered to lower the A ₁ transformation point. There is. However, if it is less than 0.05%, it is not possible to secure a hardness of 58 HRC in a quenched and tempered state even if C is at the upper limit of the range of the present invention. Also, when added over 0.30%,
N is a strong austenite-forming element, which causes an increase in retained austenite, lowers quenching and tempering hardness, and also causes a large amount of carbonitride to precipitate during annealing, thereby impairing workability. Further, the addition of nitrogen more than this causes a cost increase at present, such as the necessity of melting under pressure. Therefore N is 0.05 ~
0.30%.

B also has the function of improving the hardenability of the material and strengthening the grain boundaries to improve the workability during hot working. However, even if added in an amount exceeding 0.015%, the effect is saturated and grain boundary segregation becomes strong and embrittles. Therefore, the upper limit is made 0.015%. In order to secure a hardness of 58 HRC (653 HV) or more in the quenched and tempered state, the total amount of C + N must be 0.55% or more. But,
If the content exceeds 0.85%, both C and N are strong austenite forming elements, so that retained austenite increases and the quenching hardness decreases, and in the annealed state, carbonitrides become coarse and impair workability. Become. Therefore, the total amount of C + N is set to 0.55 to 0.85%.

V has a stronger tendency to form carbides than Cr does.
Further, since the carbide is very hard, it is effective for improving the quenching hardness. However, if it is added in excess of 0.50%, the carbides become coarse and the hardness in the annealed state is improved, thereby deteriorating the workability. Therefore, the upper limit is 0.5
0%. Ti has a strong tendency to form carbides as compared with Cr, and since the carbides are very hard, it is effective in improving quenching hardness. However, if it is added in excess of 0.50%, the carbides become coarse and the hardness in the annealed state is improved, thereby deteriorating the workability. Therefore, the upper limit is set to 0.50%.

W has a greater tendency to form carbides than Cr does.
Further, since the carbide is very hard, it is effective for improving the quenching hardness. However, if it is added in excess of 0.50%, the carbides become coarse and the hardness in the annealed state is improved, thereby deteriorating the workability. Therefore, the upper limit is 0.5
0%. These V, Ti, and W may be added in combination to increase quenching hardness. However, since each of them has a high tendency to form carbonitrides, the addition of a large amount exceeding 0.70% causes the addition of coarse carbonitrides. Will promote generation. Therefore, the upper limit in the case of adding composite is set to 0.70%.

[0015]

The embodiments will be described below. After melting the steel of the present invention and the comparative steel shown in Table 1 in a vacuum melting furnace,
After forging into 20 bar steel and annealing at 870 ° C., it was subjected to various tests. Each test item of an annealing hardness measurement as a workability evaluation in an annealed state, a limit upsetting ratio measurement, an induction hardening characteristic as a hardenability evaluation, and a cycle wet test as an evaluation of weatherability was performed.

[0016]

[Table 1]

Annealing hardness The T section of the material annealed at 870 ° C. was measured with a Vickers hardness tester. - the limit upsetting ratio annealed condition continue to pressure while restraining both end surfaces TP length L _0, when the height when cracking in the test one side for the first time resulting is L, the limit upsetting ratio ( %) = 1− (L−L)
₀ ) / L ₀ × 100.

Induction quenching characteristics A φ20 material is subjected to induction quenching under the conditions of a plate current of 3.1 A, a plate voltage of 9.0 kV, a moving speed of 232 mm / min, a rotation speed of 160 rpm, and a heating temperature of 1050 to 1100 ° C. An air-cooled tempering treatment was performed at 150 ° C. × 1 h. The induction hardening hardness was evaluated by measuring a portion 0.2 mm from the surface of the sample after the heat treatment with a Vickers hardness meter. For the induction hardening depth, the hardness was measured at a pitch of 0.05 mm from the end of the sample toward the center from the specimen, which had been subjected to induction hardening and tempering under the above-described conditions.
The evaluation was made based on the distance from the sample end when the value was lower than V (corresponding to about 50 HRC).

Weather resistance test A test piece subjected to induction hardening and tempering was subjected to a relative humidity of 90.
A cycle wet test in which a cycle of 20 to 50 ° C. was repeated 20 times in an environment of% was performed, and the degree of rust was evaluated by rating from the appearance of the test piece at that time. Rating: No rusting, slightly rusting (area ratio of rusting point 10% or less)
If you see △, violently rust (area ratio 10% or more)
Was evaluated as x.

Table 2 shows the results of each test. In order to exhibit excellent workability during annealing, it is important that the annealing hardness is low and that the cold workability is excellent. Inventive steel has low annealing hardness and high critical upsetting ratio, while comparative steels b, d, e, f, g, i, j and conventional steel 2 have high annealing hardness. The marginal upsetting rate is also low. This is because a large amount of carbonitride precipitates in the annealed state and the precipitates are coarsened.

As for the induction hardenability, the invention steel has an induction hardening hardness of at least 653 HV (corresponding to 58 HRC) or more and 515 HV.
Can be obtained as good as 3.50 mm or more, whereas the comparative steels a, b, d, e, f, g and the conventional steels 1, 2, and 3 have low induction hardening hardness, The induction hardening depth is also shallow. This is because the comparative steels b, d, g and the conventional steels 1, 2 have coarse carbonitrides in the annealed state.
This is because the solid solution in the matrix did not sufficiently advance by short-time heating such as induction hardening. Moreover, since the comparative steel a and the conventional steel 3 have originally small amounts of C + N, sufficient induction hardening hardness cannot be obtained. The low quench hardness of the comparative steels b and e is partly due to the increase in the amount of retained austenite.

The comparative steel h to which B is not added does not have sufficient hardenability, so that the induction hardening hardness is high but the hardening depth is lower than that of the inventive steel. Regarding weather resistance,
No rust was observed in the cycle wet test for both the inventive steel and the conventional steel, but rust was observed in Comparative Steel C having a small Cr content. Further, the reason why rust was observed in the comparative steel j was that the addition of a large amount of V and W increased the amount of carbonitride and increased the battery action with the base carbonitride. In the above description and examples, hardening has been described with respect to induction hardening. However, the hardening is not limited to induction hardening and can be performed in a short time as shown in FIG. 1 in general hardening heat treatment. Therefore, it is obvious that the present invention is applied.

[0023]

[Table 2]

[0024]

From the above, it can be seen that the present invention provides a high hardness stainless steel which is superior in hardenability to conventional high hardness stainless steel and is particularly suitable for induction hardening.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between heat treatment time and hardness (HRC).

Claims (2)

(57) [Claims] C .: 0.40 to 0.70%, Si: 0.5% or less, Mn: 0.5 to 2.0%, Ni: 0.25 to 1.0% by weight% Cr: 11 to 14%, Mo: 1.5% or less, N: 0.05 to 0.30%, B: 0.001 to 0.015%, the balance being Fe and unavoidable impurities. High hardness stainless steel with excellent hardenability. However, C + N:
0.55 to 0.85% 2. V, W, T in addition to the component composition of claim 1.
i alone is 0.50% or less, or a combined total of 0.70
% Of high hardness stainless steel having excellent hardenability.